This chapter is from the book

This chapter is from the book

Bridges

Bridges are networking devices that connect networks. Sometimes it is
necessary to divide networks into subnets to reduce the amount of traffic on
each larger subnet or for security reasons. Once divided, the bridge connects
the two subnets and manages the traffic flow between them. Today, network
switches have largely replaced bridges.

A bridge functions by blocking or forwarding data, based on the destination
MAC address written into each frame of data. If the bridge believes the
destination address is on a network other than that from which the data was
received, it can forward the data to the other networks to which it is
connected. If the address is not on the other side of the bridge, the data is
blocked from passing. Bridges "learn" the MAC addresses of devices on
connected networks by "listening" to network traffic and recording the
network from which the traffic originates. Figure
3.9 shows a representation of
a bridge.

Manual Bridge Configuration Some early bridge
implementations required you to enter the information for each device on the
network manually. Fortunately, bridges are now of the learning variety, and
manual configuration is no longer necessary.

The advantages of bridges are simple and significant. By preventing
unnecessary traffic from crossing onto other network segments, a bridge can
dramatically reduce the amount of network traffic on a segment. Bridges also
make it possible to isolate a busy network from a not-so-busy one, thereby
preventing pollution from busy nodes.

Bridge Implementation Considerations

Although implementing bridges can offer huge improvements in performance, you
must factor in a number of considerations. The first is bridge placement.
Generally, you should follow the 80/20 rule for bridge placement: 80% of the
traffic should not cross the bridge, and 20% of the traffic should be on the
other side of the bridge. The rule is easy to understand, but accurately
determining the correct location for the bridge to accommodate the rule is
another matter.

Another, potentially more serious, consideration is bridging loops, which can
be created when more than one bridge is used on a network. Multiple bridges can
provide fault tolerance or improve performance. Bridging loops occur when
multiple bridges become confused about where devices are on the network.

As an example of bridging loops, imagine that you have a network with two
bridges, as depicted in Figure
3.10. During the learning process, the north
bridge receives a packet from Interface A (step 1 in Figure
3.11) and determines
that it is for a system that is not on Network Z; therefore, the bridge forwards
the packet to Network X (step 2 in Figure
3.11). Now, the south bridge sees a
packet originating on Network X on Interface C (step 3 in Figure
3.11); because
it thinks the destination system is not on Network X, it forwards the packet to
Network Z (step 4 in Figure
3.11), where the north bridge picks it up (step 5 in
Figure 3.11). The north bridge determines that the destination system is not on
Network Z, so it forwards the packet to Network X—and the whole process
begins again.

You can work around the looping problem by using the Spanning Tree Algorithm
(STA). When STA is used, each interface on a bridge is assigned a value. As the
bridge forwards the data, the value is attached to the packet. When another
bridge sees the data, if the STA value for the interface is higher than that
assigned to its interfaces, the bridge doesn’t forward the data, thus
eliminating the possibility of a bridging loop. STA eliminates the bridging loop
but still provides the fault tolerance of having more than one bridge in place.
If the bridge with the higher STA value (sometimes referred to as the
primary bridge) fails, the other bridge continues functioning because
it becomes the bridge with the higher STA value. All this is achieved by the
Spanning Tree Protocol (STP).

NOTE

STP STP is defined in the IEEE 802.1d standard.

Types of Bridges

Three types of bridges are used in networks. You don’t need detailed
knowledge of how each bridge works, but you should have an overview:

Transparent bridge—A transparent bridge is
invisible to the other devices on the network. Transparent bridges perform only
the function of blocking or forwarding data based on the MAC address; the
devices on the network are oblivious to these bridges’ existence.
Transparent bridges are by far the most popular types of bridges.

Translational bridge—A translational bridge can
convert from one networking system to another. As you might have guessed, it
translates the data it receives. Translational bridges are useful for connecting
two different networks, such as Ethernet and Token Ring networks. Depending on
the direction of travel, a translational bridge can add or remove information
and fields from the frame as needed.

Source-route bridge—Source-route bridges were
designed by IBM for use on Token Ring networks. The source-route bridge derives
its name from the fact that the entire route of the frame is embedded within the
frame. This allows the bridge to make specific decisions about how the frame
should be forwarded through the network. The diminishing popularity of Token
Ring makes the chances that you’ll work with a source-route bridge very
slim.

WARNING

Identify the Bridge On the Network+ exam, you might be asked
to identify the purpose of a certain type of bridge.

As switches become ever cheaper, bridges have been overtaken by switches in
terms of both functionality and performance. Expect to be working with switches
more often than with bridges.